Try linking to druntime alone first, before you start messing
with phobos. You'll notice that the makefile for druntime builds
a libdruntime-haiku32.a library, if you modify the makefile for
Haiku. Try building some simple executables with that first
before moving on to phobos.
You need to go through all the TARGET_FREEBSD blocks in the dmd
source and all the version(FreeBSD) blocks in druntime and add
TARGET_HAIKU, version(Haiku), and the appropriate source for
Haiku inside those blocks. We can't sit here and help you do
that, unless it's something truly unusual that you're unable to
figure out. Most of these errors seem to be the result of
missing some blocks here and there.
Admittedly, porting to a new OS is not a small job, and you seem
to be going pretty fast. Keep plugging away at it and I'm sure
you'll be able to figure it out.

Try linking to druntime alone first, before you start messing
with phobos. You'll notice that the makefile for druntime
builds a libdruntime-haiku32.a library, if you modify the
makefile for Haiku. Try building some simple executables with
that first before moving on to phobos.
You need to go through all the TARGET_FREEBSD blocks in the dmd
source and all the version(FreeBSD) blocks in druntime and add
TARGET_HAIKU, version(Haiku), and the appropriate source for
Haiku inside those blocks. We can't sit here and help you do
that, unless it's something truly unusual that you're unable to
figure out. Most of these errors seem to be the result of
missing some blocks here and there.
Admittedly, porting to a new OS is not a small job, and you
seem to be going pretty fast. Keep plugging away at it and I'm
sure you'll be able to figure it out.

Those are linker errors, because you are missing symbols like
__stdoutp and __stderrp. My guess is that you simply cut and
pasted the stdc.stdio block from FreeBSD, but Haiku likely
doesn't define the same symbols. You need to look at stdio.h and
other header files in Haiku and fill those druntime blocks in
with the declarations appropriate to Haiku. It also looks like
you messed something up with setting up rt.sections_haiku.
Finally, as Jacob mentioned, you should look at the flags that
dmd normally uses when it links, as you're missing some of those.